The invention is directed to a method for forming a Bragg grating in an optical waveguide fiber. More particularly, the novel method includes steps which minimize birefringence in the grating.
The sensitivity of optical waveguide fibers to light of certain wavelength and intensity has been known since the late 1970's. It was found that the loss characteristic and refractive index of a waveguide fiber could be permanently changed by exposing the waveguide to light of a given wavelength and intensity. A publication which describes the effect and how it may be used is, "Light-sensitive optical fibers and planar waveguides", Kashyap et al., BT Techno., 1, Vol. 11, No. 2, Apr. 1993. The publication discusses the making of light-induced reflection gratings, page 150, section 2.1, and notes that the amount of refractive index change increases as light wavelength is reduced from 600 nm to 240 nm, where the photosensitivity of the waveguide appears to peak.
In "Bragg grating formation and germanosilicate fiber photosensitivity", SPIE V. 1516, Intn'l Workshop on Photoinduced Self-Organization Effects In Optical Fiber, Meltz et al., 1991, the mechanism and magnitude of photosensitivity is discussed (page 185, first paragraph, section 1.). This publication also discusses an interferometric technique of writing gratings (pp. 185-6, section 2.) At page 189, first paragraph, a measurement of induced birefringence is presented. See also FIG. 6 of that publication.
Another publication, "Characterization of UV-induced birefringence in photosensitive Ge-doped silica optical fibers", Erdogan et al., J. Opt. Soc. Am. B/V.11, No. 10, Oct 1994, notes the dependence of induced birefringence on the orientation of the polarization direction of the light incident upon the waveguide fiber. In particular, data presented in the publication shows that the induced birefringence is greatest when the polarization direction is oriented perpendicular to the long axis of the fiber and least when the polarization direction is parallel to the long axis of the fiber. See FIG. 3a. and FIG. 4. of the publication.
The Erdogan et al. publication points out that the induced birefringence polarization anisotropy can be used to make such devices, "as polarization mode converters and rocking filters", page 2100, first paragraph. However, in devices using resonant propagation, "the birefringence can result in substantial polarization dependence of resonant grating properties, such as reflectivity", page 2100, first paragraph.
The Erdogan, et al., data shows that even in the configuration where the polarization direction is along the long axis of the waveguide, some birefringence is still induced in the waveguide. Comparing the curves of FIG. 3a. and FIG. 4., the non-polarization dependent induced birefringence is a factor in the range of about 4 to 12 smaller than the polarization dependent induced birefringence. However, even this smaller amount of birefringence is undesirable. A more versatile and effective grating would result from a writing method which produces a grating having minimal birefringence.